1. Field of Invention
The present invention relates to a method of fabricating a group III-nitride semiconductor.
2. Related Art
Group III-nitride semiconductors have been widely used in UV and blue to green light emitting diodes and short wavelength laser diode applications. They are also a very important material in high electron mobility devices. Group-III nitride is typically provided in an epitaxial film formed by an epitaxial growing process. The most popular growth method for the group III-nitride epitaxial film is by vapor-phase synthesis, for example the metal organic chemical vapor deposition (MOCVD), hydride vapor-phase epitaxy (HVPE), molecular-beam epitaxy (MBE), metal-organic chloride (MOC), and the likes. The group III-nitride film is often hetero-epitaxially grown on the substrates with different materials, such as the sapphire substrate, silicon carbide (SiC) substrate or silicon (Si) substrate, due to that the large size group III-nitride substrates are not available. The epitaxial film often has high defect density because of the mismatched lattice constants and thermal expansion coefficients between the substrate and the epitaxial film. The presence of defects of the epitaxial film can seriously affect the performance of the devices subsequently built on the substrate. Therefore, it is very desirable to reduce defect density of the epitaxial film. From application point of view, the substrate used for growing group III-nitride film may also have some undesired properties, such as the low thermal conductivity, bad electrical conductivity, or resistance for cleaving. These undesired properties seriously limit the device application and/or mass production. It is therefore desirable in some applications to remove the group III-nitride film or device from the original growth substrate for performing the following fabrication processes.
Among all group III-nitride materials, gallium nitride (GaN) is the most popular one in the semiconductor application. There are several epitaxial growth methods to improve crystal quality of the GaN film. A very commonly used one is the epitaxial lateral overgrowth technique (ELOG). Strips of the SiO2 film in specific crystal orientation direction are deposited on GaN epitaxial surface, followed by epitaxially over growth. The epitaxial layer above the SiO2 strips region has significantly lower the defect density. The threading dislocation defects are however still high at window regions and coalescent boundaries. The useable low defect density area is limited by the size of the patterned SiO2 film area. The width of SiO2 strip can not be too large. In practice, the size of the SiO2 film is typically in the range of a few micro meters. Otherwise, it is difficult for epitaxially growing to fully cover the whole surface of the SiO2 film.
There have been also various methods to separate the sapphire substrate from the GaN film. These methods includes the mechanical grinding method for mechanically grinding substrate away from the GaN film, the laser ablation method for using laser to lift off the GaN film from substrate (interfacial decomposition), or the chemical etching method for chemically etching away the substrate. However, the mechanical grinding method is time consuming and requires precise handling to achieve large area uniformity. The laser ablation method includes sequential processes, and only a small area can be lifted off in each process, so that it is also a time consuming process and needs an expansive UV laser apparatus. The chemical etching method, either wet or dry etching, is a difficult and slow process because the sapphire substrate is chemical relatively inert material.
Therefore, it is an important subject of the present invention to provide a method of fabricating a group III-nitride semiconductor with lower defect density.